Magnetic resonance imaging (MRI) apparatuses are medical diagnostic imaging apparatuses with which a radio frequency magnetic field and a gradient magnetic field are applied to a subject placed in a static magnetic field, and signals generated from the subject by the nuclear magnetic resonance are measured and used to construct images. In the MRI apparatuses, in general, a slice gradient magnetic field for determining a section to be imaged (imaging section) and an excitation pulse for exciting magnetization in the imaging section are simultaneously applied, and nuclear magnetic resonance signals (echoes) generated when the excited magnetization converges are obtained. The measured echoes are arranged in the k-space defined with a horizontal axis kx and a vertical axis ky, subjected to inverse Fourier transform, and thereby reconstructed into an image.
At the time of the echo measurement, in order to give positional information to the magnetization, a phase encoding gradient magnetic field pulse and a readout gradient magnetic field pulse are applied in perpendicular directions in the imaging plane. In order to give accurate positional information, application times and intensities of both the gradient magnetic field pulses must be accurately controlled.
The radio frequency magnetic field pulse for generating echoes and the gradient magnetic field pulse are applied in accordance with an imaging sequence set beforehand. As this imaging sequence, various kinds of sequences are known for various purposes. High-speed imaging methods enabling imaging in a short time include EPI (echo-planar imaging), BASG (balanced steady-state acquisition with rewound gradient echo), and so forth.
EPI is a method of realizing high-speed measurement by repeatedly applying a readout gradient magnetic field pulse with reversing the polarity thereof at a cycle of about 2 ms to continuously generate echoes. BASG is a method of realizing high-speed measurement by repeating excitation of magnetization and measurement of echo at a cycle of several milliseconds.
In such high-speed imaging methods, a phase encoding pulse is applied in a short time, and then echo is measured during or immediately after rise of the readout gradient magnetic field pulse in order to shorten the imaging time. For the design of the pulse sequence, the readout gradient magnetic field pulse is assumed to have a rectangular shape, and an image is reconstructed from the echoes arranged in the k-space on the basis of the above assumption.
However, waveform of the readout gradient magnetic field pulse is actually distorted during or immediately after the rise of the pulse. This is caused by output characteristics of a gradient magnetic field amplifier itself, inductance of a gradient coil, eddy current induced by application of the gradient magnetic field pulse, and so forth. With such distortion of the waveform of the readout gradient magnetic field pulse, arrangement positions of the echoes in the k-space shifts from the intended positions, and so-called distortion of the k-space is thereby generated to generate artifacts such as distortion and ghost in reconstructed images.
As a technique for avoiding this distortion of the k-space, there is a method of measuring waveform of the readout gradient magnetic field pulse, calculating distortion of the k-space in the readout direction from the obtained waveform, and correcting the distortion (refer to, for example, Patent document 1). Specifically, in EPI, echoes are repeatedly measured with making the phase encoding gradient magnetic field pulse zero and changing time integral value of the dephase pulse of the readout gradient magnetic field. Further, on the basis of time shift amounts of the individual echoes, shapes of individual readout gradient magnetic field pulses are calculated, and distortion of the k-space in the readout direction is calculated. By correcting the calculated distortion, an image is reconstructed with suppressing artifacts generated by the distortion of waveform of the readout gradient magnetic field pulse.